An acoustic wave device is used as a filter or a duplexer of a mobile communication unit. An acoustic wave element such as a surface acoustic wave element or a thin-film piezoelectric resonance element is used as the acoustic wave device. In order to downsize the acoustic wave device, a substrate on which an acoustic wave element is formed is sealed by a resin. When the acoustic wave element is sealed by the resin, a cavity is formed on a functional portion of the acoustic wave element. The functional portion of the acoustic wave element is an electrode such as an IDT (Interdigital Transducer) in the case of the surface acoustic wave element, and is an overlapping portion of an upper electrode and a lower electrode sandwiching a piezoelectric thin film in the case of the thin-film piezoelectric resonance element.
Japanese Patent Application Publication No. 2008-227748 discloses that a frame is formed on a substrate so as to surround a functional portion of an acoustic wave element, and a roof portion of the frame is formed by a photosensitive resin and a metal layer so that a cavity is formed on the functional portion. Japanese Patent Application Publication No. 2005-175345 discloses that a cavity is formed on a functional portion by bonding a resin sheet, on which a recess portion to be a cavity is formed, to a substrate on which an acoustic wave element is formed.